Active region prominence
Streamers of solar matter that can reach an altitude of 10,000 kilometers. The magnetic field in a prominence can be as high as about 50 Gauss (5 x 10-3 T) (see filament).
Field of astronomy concerning the specific study of the intermediate and high planetary atmospheres.
Fraction of the light and energy received that is reflected or diffused by a non-luminous body. The albedo is always comprised between 0 and 1. It varies according to the wavelength. An albedo equal to zero at a given wavelength characterizes a body that absorbs all this radiation perfectly. A value of 1 characterizes a perfect mirror for that wavelength.
Aligned current
Electric current aligned on the magnetic field lines. In the ionosphere, they can be found at high latitudes, in the auroral oval.
Andromeda nebula
A galaxy similar to ours in our local cluster (the Milky Way).
Index of magnetic activity (Planetary Amplitude)
Furthest point from the Sun in the orbit of a body revolving around the Sun.
Furthest point from the Earth in the orbit of a body revolving around the Earth.
Archimedes spiral
Geometric figure, the typical example of which is the trajectory described by water spraying out of a rotating garden sprinkler.
A small solar system object in orbit around the sun composed mostly of rock. Many of these objects orbit the sun between Mars and Jupiter. Their size can range anywhere from 10 meters in diameter to almost 1000 kilometers.
Astronomical Unit
Average distance between the Sun and the Earth, i.e. 1,495,978,710 meters.
Auroral oval
Zone in which ionized particles are precipitated from the magnetosphere toward the thermosphere. At all times there is one oval in the North and another in the South. It generally ranges from 65° to 75° in latitude, but can spread during magnetic storms or substorms.
BASS 2000
BAse de Données Solaires Sol 2000, Solar database dedicated to solar observations obtained on the one hand via instruments at ground level and, on the other hand, through simulations or digital extrapolations concerning solar physics. (
Big Bang
According to the 'standard' cosmological model, a simultaneous explosion throughout space that is at the origin of our universe.
Belgian Institute for Space Aeronomy (
Blue streamer
Blue flash connecting the low atmosphere and the terrestrial ionosphere.
Brown dwarf
Star whose mass is too low for hydrogen fusion to take place.
Centre de Données de la Physique des Plasmas : Plasma Physics Data Centre. (
Coronal Diagnostics Spectrometer, a solar observation instrument on the SOHO satellite. (
Region of stellar, particularly solar, atmosphere, above the photosphere and below the corona, characterized by a sudden increase in temperature. During a total eclipse of the Sun it shows up as a thin, bright pink layer, hence its name. The color is due primarily to the emission of hydrogen at 656.3 nanometers. On the Sun, it has a depth of approximately 10,000 kilometers.
Chromospheric plage
Zone of the chromosphere in which the field lines of a spot favor the dissipation of energy toward the immediate atmosphere, giving rise to hot atmospheric zones that can reach into the solar corona.
Circular orbit
Satellite orbit whose eccentricity is close to zero.
Constellation of four satellites launched in 2000 to study the terrestrial magnetosphere. This is a ESA/NASA project. (
Coronal Mass Ejection
Solar system body, consisting of a solid nucleus made up of rock and ice. There are two comet sources: the Oort Cloud and the Kuiper Belt. As they near the Sun, comets give off an atmosphere in the form of two tails, one neutral and one ionized.
Transfer of thermal energy via fluid currents (gases or liquids).
Convection zone
External region of the inside of the Sun, where the energy produced by the nuclear core is transmitted by convection.
Optical instrument simulating an eclipse.
Coronal hole
Region of the corona where the magnetic field lines open into space allowing the solar wind to escape.
Coronal mass ejection
Sudden mass ejection from the Sun, in the widest sense.
Coronal streamer
Ejection of solar gas from the corona.
Joint rotation of the atmosphere and a planet.
Cosmic ray
High energy sub-atomic particles from outer space that collide with the earth's atmosphere to produce a shower of gamma rays (and other particles) at high energies.
Cross-tail current
Electric current created by the separation of charges in the magnetosphere tail (night side of the Earth).
Current ring
Zone of the magnetosphere, at an altitude of more than four terrestrial radii, through which ions and electrons travel due to the combined effect of the gravitational and geomagnetic fields.
Arc of a meridian between a point on the surface and the point on the equator.
Parameter of an ellipse that characterizes its flatness. Eccentricity is equal to 1 in the case of a straight line and to 0 in the case of a circle.
Plane in which the Earth orbits the Sun.
Extra High Frequency, radio wave whose frequency is between 30 and 300 gigahertz.
Extreme ultraviolet Imaging Telescope, solar observation instrument on the SOHO satellite. (
Extra Low Frequency, radio wave whose frequency is between 10 hertz and 3 kilohertz.
Elliptic orbit
Satellite orbit with excentricity greater than 0 and smaller than 1.
Equatorial orbit
Satellite orbit whose inclination is close to zero.
Homogenous substance used, alone or with other substances in the composition of a propellant.
European Space Agency (
European Space WEather Portal
Upper zone of the heterosphere where the temperature is constant.
Exospheric temperature
Temperature of the thermosphere above 400 kilometers.
Decimetric index measurement of the solar emission flux at 10.7 cm (expressed in 1022 W m-2 Hz-1)
A region or spot that is brighter than the rest of the solar surface.
Fast solar wind
Solar wind ejected above the coronal holes or during eruptions (speeds of roughly 700 to 950 km s-1).
Dark, elongated structure on the solar corona, that probably forms a boundary with the chromospheric magnetic field.
Long structure that can be seen on the Sun near magnetic field reversal lines. When observed against the bright surface of the Sun, a filament appears darker since it is colder. When observed on the limb of the Sun, as it passes through the less luminous solar atmosphere, it appears to be bright (see prominence).
Faint Object Camera on board the Hubble Space Telescope (ESA).
Group of stars, dust and interstellar gas, isolated in space, whose cohesion is maintained by gravity.
Gas giant
Refers to Jupiter, Saturn, Uranus and Neptune.
Geostationary Earth Orbit, see geostationary
Geographic pole
Intersection between the rotation axis of any heavenly body and its surface.
Geomagnetic field
Magnetic field of the Earth.
Geostationary orbit
Satellite orbit that always flies over the same point of the terrestrial equator. Geostationary satellites are on what is called a 'high orbit', at 36,000 kilometers.
Geosynchronous orbit
Orbit in which satellites orbit with the same frequency as the movement of the Earth around its axis.
Geomagnetically Induced Current
Luminous structure on the solar photosphere, that shows up as a cell with a mean diameter of 1200 kilometers and an average life span of 18 minutes. In between granulations, the matter appears darker. These granulations correspond to the seething of the convective zone.
Goddard Space Flight Center, a NASA center (
Region in space that undergoes the influence of the solar wind. It spreads out approximately 50 to 100 AU from the Sun.
Heliosynchronous orbit
Low orbit in which a satellite always passes over a node at the same solar time.
Highly Eccentric Orbit, see Molnya
Zone of the atmosphere typically situated above 80 kilometers, where each of the constituants has its own scale height since each of them has its own pressure decrease rate.
High Frequency: radio wave whose frequency is between 3 and 30 megahertz
High orbit
See geostationary
Upper boundary of the homosphere, at an altitude of about 80 kilometers
All the lower layers of the terrestrial atmosphere in which the scale height is the same for all the constituants: and the pressure and concentration decrease rates follow the same law for all. The homosphere ends at an altitude of about 80 kilometers.
International Geomagnetic Reference Field, an empiric model of the geomagnetic field. (
Angle between the plane in which a body orbits and the equatorial plane.
Intermediate orbit
Satellite orbit at about 20,000 kilometers from the Earth.
Interplanetary magnetic field
Magnetic field transported by the solar wind
Gas consisting of charged particles (ions and electrons) that mingle with the thermosphere to form the atmosphere of high altitudes.
Institute of Space and Astronautical Science, Japanese space and space weather agency (
International Solar Terrestrial Physics. A program created jointly in 1980 by three space agencies, NASA, ESA and ISAS. The aims of the ISTP are essentially scientific: to determine the structure and dynamics of the interior of the Sun, understand solar activity, corona heating, the acceleration of the solar wind, the interplanetary medium, the impact on the terrestrial atmosphere and the magnetosphere. This program develops space instruments and theoretical models.(
See unit of solar flux.
Kinetic pressure
Pressure exerted by a gas.
Index of magnetic activity
Kuiper Belt
A second source of comets and small solar system bodies, in the shape of a flat ring stretching from the orbits of Neptune and Pluto to the internal boundary of the Oort Cloud. It is thought that it could contain more than ten times as many comet nuclei as the Oort Cloud.
L1 halo
Satellite orbit around the L1 Lagrange point. Also known as L1 Lissajous.
L1 Lissajous orbit: see L1 halo.
Lagrange point
Zone in space where the gravity of two bodies balances out. Here we consider point L1 which is between the two bodies.
Angular distance from the equator in a system of spherical coordinates.
Legal time
Time defined by each country.
Low Earth Orbit, see low (orbit)
Low Frequency, radio wave whose frequency is between 30 and 300 kilohertz
Luminous edge of the disk of a heavenly body.
Local time
See LT.
Angular distance along a parallel, calculated from a source meridian in a system of spherical coordinates.
Low orbit
Satellite orbit at an altitude of about 1000 kilometers above the Earth.
Lockheed Martin Palo Alto Research Lab. (
Local Time, the time in relation to the Sun where the observer is located
Solar matter ejected permanently between the supergranules, in the form of proton and electron tongues.
Magnetic cloud
Front of a coronal mass ejection, once it has reached space.
Magnetic declination
Angle between the magnetic meridian and the geographic meridian at a given point.
Magnetic mirror
Phenomenon that transfers energy related to movement that is parallel to the field lines into energy related to perpendicular movement; as a result, the particles slow down, stop and then reverse when they enter the zones where the field lines close in.
Magnetic pole
Region on a body where magnetic inclination is at a maximum.
Magnetic pressure
Pressure exerted by magnetic forces.
Magnetic storm
Considerable perturbation of the geomagnetic field due to its interaction with the interplanetary magnetic field. Storm: the effect is planetary. Sub-storm: it is confined to high altitudes.
Boundary between the magnetosphere and the solar wind.
Magnetopause current
Current created by the separation of solar wind charges on the magnetopause.
Zone on the terrestrial magnetic field where the solar wind is compressed.
Cavity inside which the plasma is subjected to the geomagnetic field and relatively protected from the solar wind.
Maunder minimum
Period during which practically no sunspots were observed (from 1645 to 1715).
Michelson Doppler Interferometer. This is an optical instrument (interferometer) on board the SOHO satellite, that takes Doppler pictures of the Sun. (
SOHO data and control center. (
Medium Earth Orbit, see medium (orbit)
A line connecting all points are the same longitude on the surface of the Earth or on any other heavenly body.
Boundary between the mesosphere and the thermosphere.
Region of the atmosphere situated at an altitude of approximately 50 to 80 kilometers.
Meteoroid as it enters the terrestrial atmosphere where it heats, triggering the shooting star phenomenon.
Fragment of a meteor found on the ground.
A small solid object orbiting the Sun. Typical meteoroids have a mass below 100kg.
Medium Frequency, radio wave whose frequency is between 300 kilohertz and 3 megahertz.
See SHF.
Milky Way
Name of our galaxy.
Satellite orbit describing a very flat ellipse (a perigee of 400 kilometers and an apogee of 40,000 kilometers, approximately). Named after a Russion military communications satellite system.
Mass-Spectrometer-Incoherent-Scatter model, or international model of the high terrestrial atmosphere. The MSIS-E version also covers the ground. (
National Aeronautics and Space Administration; American space agency, (
Neutral sheet
Zone of the magnetopause, on the night side, where the solar wind ions and electrons meet up. Also know as Plasma Sheet.
Fundamental particle with no electric charge and very low mass.
National Geophysical Data Center, in the USA. (
North American Aerospace Defense Command. This is a binational (USA-Canada) control center, charged with warning of attack by aircraft (or missile) of either of these countries and organizing air defense in the case of such an attack. (
Formation of chemical elements by nuclear reaction, typically occuring in the core of stars or in the first few moments after the big-bang.
Oort Cloud
One of the comet sources, on the border of the solar system — between 20,000 and 50,000 AU1 - . It is a tenuous sphere whose existence was first conjectured in the 1950s. It is estimated that this cloud contains between 100 and 1000 billion comet nuclei.

1 This represents slightly less than the distance between the Sun and the nearest star, Proxima Centauri.

Molecule consisting of three atoms of oxygen.
A line connecting all points at the same latitude on the surface of the Earth or of any other body.
The point in its orbit where a satellite traveling round the Earth is closest to the Earth.
The point in its orbit where a body traveling round the Sun is closest to the Sun.
Individual particle of electromagnetic energy that makes up electromagnetic radiation.
Surface of the Sun that is visible in white light. The major part of solar radiation comes from the photosphere and a very small part from the corona.
Planetary nebula
Cloud of gas ejected from a red giant star, the nucleus of which is left behind as a white dwarf star. The name was given by 18th century astronomers because of the visual appearance of the brightest planetary nebulae, which looked like faint planet disks through the telescopes of the era. Despite the name, there is no physical relationship between planets and planetary nebulae.
Gas in which charged particles (ions, electrons), are free. In geophysical plasmas, the charges are neutral, i.e. there are just about as many electrons as there are monovalent ions.
External boundary of the plasmasphere.
Zone of the magnetosphere that co-rotates with the Earth.
Streamers of solar matter above the coronal holes.
Polar aurora
Luminous atmospheric phenomenon (boreal in the northern hemisphere, austral in the southern hemisphere) due to the de-energizing of atmospheric gas excited by collisions with solar wind particles.
Polar circle
Parallel beyond which the Sun does not rise at least one day per year (or does not set at least one day per year).
Polar cusp
Zone of the magnetosphere where the geomagnetic field lines open out onto the magnetosheath and into space.
Polar orbit
Satellite orbit with an inclination of 90°, thereby crossing both poles once every orbit.
Antiparticle to the electron, with a positive charge.
Product composed of one or several ergols, capable of producing the energy required to propel a rocket engine, via chemical reaction.
Quasi-equatorial orbit
Orbit with an inclination greater than 0°, but smaller than 10°.
Quasi-polar orbit
Satellite orbit with an inclination between 80° and 100°.
Quiescent prominence
Quiet Sun prominence, at an altitude of up to 100,000 kilometers and a magnetic field of about ten Gauss (10-3 T).
Radiative zone
Internal region of the Sun, between the nuclear oven and the convection zone. Here, the energy produced by the nuclear core is transmitted by radiation.
Reconnection point
Zone of the magnetosphere, on the night side, where the plasma sheet encounters the magnetopause.
Red giant
Star in which hydrogen combustion in the nuclear core has ceased; the helium heart becomes denser and hotter that it was originally and the shell dilates (up to about 100 times its original size).
Index of monthly sunspots
Average index of monthly sunspots
Ring current
An electric current that creates a ring at a low latitude around the Earth, at between four and seven terrestrial radii from the surface of the planet.
Schwabe cycle
Cycle of solar activity, lasting between 10 and 12 years.
Super High Frequency, radio whose frequency is between 3 and 30 gigahertz
In the field of aeronomy, name sometimes given to the solar side of the magnetopause.
Sidereal rotation
Rotation of the Sun (or a heavenly body) observed relative to a fixed point outside the solar system (e.g. a star)
The first American space station (
Slow Solar wind
Solar wind ejected above the quiet photosphere (speeds of roughly 300 to 450 km s-1).
SOlar Heliospheric Observatory (
Solar constant
Total solar power received by a surface one meter square, perpendicular to the solar radiation, at a distance of 1 AU (1366.1 W m-2).
Solar corona
High temperature region of the solar atmosphere, above the chromosphere; here, plasma is trapped by the local magnetic field or escapes into space if the magnetic field lines are open.
Solar eruption
Mass ejection from the Sun, related to a prominence.
Solar flux unit
Unit used to measure the solar flux, especially the f10.7 index (in 1022 W m-2 Hz-1)
Solar wind
Particle flux related to the interplanetary magnetic field. It is composed primarily of electrons and protons (expelled continuously from the Sun toward space).
Space weather
Space weather is the physical and phenomenological state of natural space environments. The associated discipline aims, through observation, monitoring, analysis and modelling, at understanding and predicting the state of the sun, the interplanetary and planetary environments, and the solar and non-solar driven perturbations that affect them; and also at forecasting and nowcasting the possible impacts on biological and technological systems
Solar matter expelled permanently between the granules, in the form of proton and electron tongues.
Red flash linking the low atmosphere and the ionosphere of the Earth.
Luminous gaseous heavenly body, whose energy is normally provided by nuclear fusion reactions taking place in its core. Objects where nuclear fusion no longer takes place but did in the past are also considered stars, as are brown dwarfs, objects heavier than planets, but too light to sustain fusion reactions.
Solar TErrestrial RElations Observatory. A Nasa mission consisting of two nearly identical space-based observatories - one ahead of Earth in its orbit, the other trailing behind - intended to provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections. (
Upper boundary of the stratosphere.
Region of the atmosphere between the troposphere and the mesosphere, at an approximate altitude of 12 to 50 kilometers.
Radio wave with a frequency of between 300 gigahertz and 1 terahertz.
Subsolar point
Area of the magnetosphere situated on the imaginary line between the center of the Sun and the center of the Earth.
Dark zone of the photosphere, with a mean diameter approaching a few thousand kilometers. The spots are areas with a strong magnetic field and are dark because they are colder than the photosphere.
Large-size granulation (30,000 kilometers).
Stage in the life of a massive star, manifesting itself by an explosion that makes it extremely bright for a time.
Space WEather European NETwork
Synodic rotation
Rotation of the Sun observed from the Earth, i.e. taking into account the Earth's own rotation (on its axis and around the Sun).
Terrestrial planet
Refers to the solar system planets which are similar to the Earth in size and composition: Mercury, Venus and Mars. Also know as Telluric planets
Telescopio Heliográfico para el Estudio del Magnetismo y de las Inestabilidades Solares. Heliogaphic Telescope for the Study of Magnetism and Solar Instabilities (
Zone where the temperature of the neutral terrestrial atmosphere increases, at an altitude exceeding 80 kilometers, approximately.
Transition Region and Coronal Explorer. Small satellite developed by the Lockheed Institute in Stanford (USA) to observe the evolution of the magnetic structures of the Sun. (
Transition region
Low solar corona characterized by a sudden increase in temperature (from a few thousand to a few million degrees).
Transition zone
Region of the solar atmosphere, between approximately 3000 kilometers and forty thousand kilometers, where the temperature increases from 10,000 degrees to more than one million degrees.
Region of the atmosphere closest to the ground, whose depth increases between the pole (8 kilometers) and the equator (17 kilometers).
Ultra High Frequency; radio wave whose frequency is between 300 megahertz and 3 gigaherz.
Satellite developed to explore the region in space that is exposed to the solar wind, by observing it at all the solar latitudes for the first time (ESA / NASA) (
Universal time
See UT
Ultraviolet Coronagraph Spectrometer onboard SOHO (
Van Allen Belts
Zones in the magnetosphere surrounding the Earth in which highly energized particles are trapped.
Very High Frequency, radio wave whose frequency is between 30 and 300 megahertz
Very Low Frequency, radio wave whose frequency is between 3 and 30 kilohertz
Wide-Field and Planetary Camera 2 on the Hubble space telescope (NASA).
White dwarf
Phase of a solar-type star following that of a red giant. The surface temperature of a white dwarf is around 10,000 Kelvin and its luminosity is very low. Average voluminal mass is approximately 1 ton per cubic meter.
NASA satellite for observing the solar wind, located near the Lagrange L1 point. (
Japanese solar observation satellite that operated since December 2001 in the X-ray and gamma ray ranges (